The present invention relates to a driving circuit for a magnetic clutch which operates to couple a refrigerant compressor used in an air conditioner for automotive vehicles, to a vehicle engine to allow transmission of torque from the latter to the former.
A refrigerant compressor, which is used in an air conditioner for automotive vehicles, is generally provided with a magnetic clutch which comprises a driving member such as a pulley which engages with a driver such as a vehicle engine, through a belt or the like, a clutch plate arranged close to a side surface of the driving member and secured to the main shaft of the compressor, and a magnetic coil arranged on the other side surface of the driving member, whereby energization of the magnetic coil causes frictional engagement of the clutch plate with the driving member to allow transmission of torque produced by the driver to the main shaft of the compressor.
In the above magnetic clutch, magnetomotive force or electromagnetic force, which is produced in the magnetic coil by electric current flowing therethrough, is always substantially constant, whereas torque, which is transmitted from the driving member to the clutch plate, is relatively large at the instant of engagement of the clutch and thereafter is relatively small. Therefore, a large amount of electromagnetic force is required at the instant of engagement of the clutch while just a small amount of electromagnetic force is sufficient for keeping the engagement of the clutch after the engagement of the clutch is completed. However, according to the conventional magnetic clutch, a larger amount of electric current than that required always flows through the magnetic coil during operation of the refrigerant compressor after engagement of the clutch, which results in early exhaustion of the battery used as the power source and thermal deterioration of the magnetic coil.